Fukushima Gov. Yuhei Sato (orange helmet) inspects the contaminated water tanks at Tokyo Electric Power Co Fukushima Dai-ichi nuclear power plant at Okuma town in Fukushima prefecture on Oct. 15.

In an alarming Friday-morning alert, the operator of the stricken Fukushima Daiichi nuclear-power plant said that radiation levels on Oct. 17 had spiked some 6,500 times higher overnight, at a well near the spot where 300 tons of highly radioactive water is thought to have leaked in August.

Where that 300 tons of water went has always been unclear, since there was no physical trace of it in the area. The well had been dug to check whether there were any signs that contaminated water was spreading from the leak site, and the spike in radioactivity – a day after Typhoon Wipha dumped masses of rain on the plant – suggests that it was starting to, said Tokyo Electric Power Co.

It’s a lot harder to figure out just how bad that creeping contamination is, however. JRT’s preliminary answer: It could be quite bad, since Tepco’s data suggests the water could contain dangerous strontium-90 at a level thousands of times higher than the legal emission limit.

Tepco tests water at many of Fukushima Daiichi’s wells daily, using a quick method that measures something it calls “zen-beta’’ in Japanese – or “all-beta’’ in English. That’s a measure of how much beta radiation there is in the sample, using a unit called becquerels (Bq) that tracks the energy released per second.

Friday’s alert said that the zen-beta in the contaminated well was 400,000 Bq per liter – a record-high measurement.

What’s emitting that beta radiation makes a big difference, however. Beta-emitter tritium, for instance, is a radioactive form of hydrogen seen as one of the least harmful elements – while strontium-90 is a much more dangerous element linked to bone cancer. Nuclear plants in Japan are allowed to release 60,000 Bq per liter of tritium, but only 30 Bq per liter of strontium-90.

So how much of that 400,000 Bq per liter is comprised of what? Tepco says it doesn’t know since it takes a lot of time to do that analysis.

Still, it turns out that none of it is tritium, since the type of quick measurement used for the beta-radiation check isn’t sensitive enough to pick up tritium, says a Tepco spokesman.

What’s more, Tepco has found that the zen-beta from the kind of water that was stored in the leaky tank is generally half from strontium-90 and half from a radioactive form of an element called yttrium, which is formed from strontium-90 in the process of nuclear decay.

That would suggest there could be 200,000 Bq per liter of strontium-90 in the well – more than 6,600 times the allowed emission limit.

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Japan Real Time is a newsy, concise guide to what works, what doesn’t and why in the one-time poster child for Asian development, as it struggles to keep pace with faster-growing neighbors while competing with Europe for Michelin-rated restaurants. Drawing on the expertise of The Wall Street Journal and Dow Jones Newswires, the site provides an inside track on business, politics and lifestyle in Japan as it comes to terms with being overtaken by China as the world’s second-biggest economy. You can contact the editors at japanrealtime@wsj.com